Six weeks after the tsunami that may have killed 300,000 people on the shores of the Indian Ocean, scientists are discovering more about the changes wrought by the magnitude 9 quake, the fourth-largest in the last century.

It caused upheaval on the sea floor near its epicentre off the northwest coast of Indonesia's Sumatra island and moved several other islands, but scientists say any movement of land mass can be measured in centimetres rather than tens of metres.

Chen Ji, a seismologist at the California Institute of Technology, said he found movement along the fault line of about 10 metres laterally and four or five metres vertically.

Inaccurate reports

But reports that the entire island of Sumatra - 1700km long and 400km wide - moved 35 metres or more are wildly inaccurate, scientists say.

"We know we have movements of over a metre, perhaps a couple of metres"

Ken Hudnut,
US Geological Survey

"We know we have movements of over a metre, perhaps a couple of metres," said Ken Hudnut, a California-based geophysicist with the US Geological Survey.

"But the idea that Sumatra has moved 100 feet (30m) is just wrong."

Scientists are working on precise measurements by comparing geographic points whose locations were known before the quake with their new positions after, using the Global Positioning System, which reads exact locations by using satellites.

High-tech British and US ships are investigating changes to the sea bed and local authorities are measuring depths in critical shipping channels.

Microseconds off schedule

Scientists at US space agency Nasa said the 26 December quake - the largest to rattle Earth since 1964 in Alaska - disrupted the planet's rotation and shaved 2.68 microseconds, or millionths of a second, from the length of a day.

Nasa scientists BF Chao and Richard Gross calculated it shifted Earth's mean north pole about 2.5cm and made the planet slightly less oblate, or flattened, at the poles.

"Physically, this is analogous to a spinning skater drawing arms closer to the body, resulting in a faster spin," they wrote in an article in Eos, a publication of the American Geophysical Union.

But they said these changes are based on calculations rather than measurements. The changes are so small they are either difficult to measure or too small to detect.

Many earthquakes shake the planet's axis and affect its rotation, scientists added, but their impact is too small to measure.

Vast damage

But environmental damage from the tsunami was vast. The killer waves gouged beaches, crushed coral reefs, smashed thousands of hectares of mangrove forests and refashioned
coastlines from Thailand to Somalia.

Up to 300,000 people may be 
dead after the tsunami disaster

A preliminary survey by the Indonesian government and the United Nations Environmental Programme (UNEP) estimated the economic cost to the environment at $675 million in Indonesia alone.

The survey said 25,000 hectares of mangroves and 29,000 hectares of coral reefs were damaged.

Some coral reefs - undersea gardens that act as shelter and nursery to a wide range of marine species - were crushed by the waves.

Corals grow slowly, some only a few centimetres a year, so their recovery could take decades.

John Pernetta, a UNEP official in Bangkok, said the extent of damage to some of the coral reefs around Thailand was very high - up to 80% in some places. Their recovery was
uncertain.

Mangroves

But mangroves torn out by the waves will fare better, he said, as they leave behind roots and seeds that will help them regenerate.

"There is evidence that a lot of sediment was being brought onshore"

Wave researcher Phil Liu,
Cornell University

"Long-term damage to mangroves by hurricanes or tsunamis doesn't really happen," Pernetta said. "After five to 10 years you don't even know anything has happened."

Vast stretches of Sumatra's west coast were turned brown by the tsunami as rice paddies and other vegetation were swamped by salt water.

It could take two or three rainy seasons to wash the salt from the saturated land, experts say.

The tsunami waves also ate away beaches and coastal areas in Thailand, Indonesia and Sri Lanka, radically changing maps.

The waves also carried sediment ashore, said Phil Liu, a Cornell University wave researcher who led a scientific team to Sri Lanka in mid-January.

"There is evidence that a lot of sediment was being brought onshore," he said. "A post office on the east coast was found with sediment deposits on the roof."

But it remains to be seen whether such sediment is good for the land or a bane because of its high salt content.